1. Field of the Invention
The present invention relates generally to data transmission systems and, more particularly, to measurement of interference in data transmission systems.
2. Description of Related Art
In transmission systems, transmitted data often is affected by concurrent transmission on the same frequency range or neighboring frequency ranges. Therefore, the design of some systems provides a plurality of transmission channels of which any channel or any subset of channels of the plurality of channels may be selected for transmission. Before a channel is chosen for transmission, interference of concurrent transmissions on that channel is measured. If the interference is considered too high, another channel is chosen. A change of the transmission channel may be also performed in the course of a transmission if the interference situation of the chosen channel worsens (i.e. systems with dynamic channel selection or re-selection). Frequency-hopping systems are also examples of systems with interference measurement. The frequency-hopping systems change the transmission channel at given time instants to achieve a statistical distribution of the interferers.
Typically, the quality of all possible channels is assessed by measurement samples of the received signal strength of the wanted transmissions and the signal strengths of interfering transmissions. In a next step, a set of interference measurements, or signal-to-interference power ratios calculated from the set of interference measurements, are usually further condensed into meaningful representative values. The meaningful representative values could, for example, be the mean of the measurements, the maximum or minimum, or the empirical distribution or samples thereof.
The condensation from the measurement samples into the meaningful representative values is especially relevant when the measuring device must report its measurements to other devices, which then may, for example, decide to perform a channel reselection or a hop set adaptation. Since the measuring-device reports compete with user traffic for the available transmission capacity, it is desirable to use only as many bits as necessary to report measurement results. If the measurements are performed at a decision-making device, it is also desirable to condense the measurement values in order to reduce the memory requirements.
Today, many digital transmission systems use packetized transmission (i.e. the information is grouped and sent within bursts). As a consequence, the transmitted power is not constant over time, but is ramped up at the beginning of a burst transmission and ramped down at the end of the burst. Further, several wireless transmission systems use time-division duplex, frequency hopping, or do not transmit power during idle times of the transmission system. For any of these reasons, the interference power in the case of interference by such systems is significantly time variant. In addition, the received power of the wanted link can be time variant, such as, for example, due to fading, especially if frequency hopping is applied.
The time variance of the received interference power is especially strong if the spatial distance to the interferer also exhibits a strong variance, as is commonly the case in uncoordinated wireless personal area systems such as BLUETOOTH. In contrast, typical cellular systems usually exhibit relatively small system internal interferer distance variations, and thus significantly smaller interference power variations.
Packet data transmission systems usually provide means to determine the integrity of the received data. Determination of the integrity of the received data is normally done by adding redundancy to the information content, such as, for example, by cyclical redundancy check (CRC) information. If the CRC succeeds at the receiver, the content of the respective packet (or packet part) protected by this CRC is considered to be valid.
As described above, it is important to condense the measurement outcomes of the interference measurements to a small number of meaningful representative values in order to reduce the amount of data to transmit per measurement report, while at the same time retain the statistical significance of the measurements. The average of the measurement samples is usually a good candidate for such a meaningful representative.